The previous tests didn't cover the case Number.isSafeInteger(x)
where TurboFan was unable to tell that `x` is always a Number and
thus had to use the ObjectIsSafeInteger operator instead.

Bug: v8:8015
Change-Id: I9bdbfa602fe0bf8c5fb2bc6c160ace7ab0bc0aaa
Reviewed-on: https://chromium-review.googlesource.com/1238234Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56114}